you need to get him to understand that you did not do it on purpose. you can get cuts on yourself without even knowing about it. If he does not understand maybe he is not someone you should be with. You should be with someone that will believe you no matter what.

If he doesn't believe you that's his problem. And he shouldn't be controlling you anyways. But he should definitely believe you if you tell him.

Well as hard as it is you need to tell him. If you keep it from him and he does not notice then its not a big deal if you wait until he notices then explain it then it might be better then telling him right away that you cut yourself by accident.
I know how you feel I have had this happen a few times to me, where I got cuts from a rose bush or from shaving in the shower the list goes on. I normally just waited until someone asked what happened and I would just tell them. I did once tell my mom that I cut myself on something when I was cleaning something out but I didnt know how or when as I didnt feel it but it was before she noticed it. And she right away didnt think it happened by mistake she thought I cut myself and there was a huge fight over it.
So I just wait and if they notice ok and if they dont then alright. you should not be making yourself crazy he should understand and trust you after all I am sure he most likely has cut himself by mistake when shaving or doing work etc.